A PCB is a thin board for mounting electrical parts on at least one of its surfaces, such as integrated circuits (ICs), resistors, switches, and the like, in order to obtain a printed circuit board assembly (PCBA). During the manufacturing process of a PCB typically a copper circuitry is formed on a non-conductive, insulating substrate/layer, which is typically made of an epoxy resin or a polyimide resin. The most common non-conductive substrate/layer is a glass-reinforced epoxy laminate, commercially known as FR-4.
In order to obtain a multi-layer PCB, typically a stack of non-conductive, insulating substrates/layers with its conductive copper circuitries is formed to obtain a one piece product.
Typically, during PCB manufacturing (in particular during manufacturing a multi-layer PCB) two particular types of non-conductive, insulating substrates/layers are utilized, which are permanent: (i) the aforementioned non-conductive, insulating substrates/layers also called build-up layers, pre-pregs, inner layers, insulating layers etc. forming within a multi-layer PCB the non-conductive, insulating substrates/layers between circuitries and (ii) solder masks (also often referred to as solder resists, outer layers etc.) covering and protecting the outer most circuitry in multi-layer and non-multi-layer PCBs.
A solder mask is, after its polymerization (also called curing or hardening), a fully polymerized, permanent layer of non-conductive material which covers the surface of the outer most non-conductive substrate/layer and most of its copper circuitry. A solder mask is patterned to fully cover the circuitry, except for those portions of the copper circuitry intended to be exposed for soldering. In such areas the solder mask typically exhibits openings, such as pad openings.
Usually, in a first step, a solder mask is formed from a photo imageable composition which is applied to the surface of the non-conductive substrate/layer to form a photo imageable cover layer. In a second step, after a brief pre-polymerization of the photo imageable cover layer at a temperature usually in the range from 70° C. to 80° C., the photo imageable cover layer is typically exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photo imageable cover layer is developed in a developer solution (an organic solvent, an aqueous, or semi-aqueous solution) which washes away unexposed portions of the cover layer. Afterwards, a photo imageable cover layer with openings remains, which is fully polymerized at a temperature typically in the range from 150° C. to 200° C., to form a non-conductive, fully polymerized, patterned, and permanent solder mask intended to protect the circuitry on the outer surface of the PCB for its entire life time.
Afterwards, the PCB is subjected to a soldering process and a metal or metal alloy is deposited into said openings. A typical procedure is a metal deposition out of a metal containing electrolyte by means of electroless metal deposition. A commonly deposited metal is tin. However, according to our own experiments such metal containing electrolytes often show a tendency to infiltrate the non-conductive, fully polymerized, patterned, and permanent solder mask especially at the edges of the openings. As a result, in the vicinity of the openings the metal is sometimes deposited underneath the solder mask, mainly between the solder mask and the surface of the copper circuitry. It is assumed that insufficient adhesion (and along with it a certain delamination) between the solder mask around an opening and the respective copper surface of the copper circuitry is the reason of this infiltration.
U.S. Pat. No. 5,484,688 A discloses in its abstract “a process for the patterned metallisation of structured printed circuit boards in which the fully structured printed circuit board is covered with a solder stop mask, with the solder contact locations being left open, the solder stop mask is heated under such conditions that complete hardening does not yet occur and the copper surface of the printed circuit board is practically not oxidized, metal is deposited out of an aqueous bath at the exposed solder contact locations and after the metal deposit operation the mask is heated sufficiently long to a sufficiently high temperature that the mask is completely hardened throughout. The incomplete pre-hardening step prevents the solution infiltrating under the mask in the metallisation operation and thereby loosening the adhesion thereof.”
The process disclosed in U.S. Pat. No. 5,484,688 A is a two-step process, including a pre-polymerization step of the solder mask prior to soldering and a post-polymerization step after soldering in order to obtain a fully polymerized, patterned, and permanent solder mask. According to claims 7 and 8 in U.S. Pat. No. 5,484,688 A “the solder stop mask [ . . . ] [is] heated prior to the metal deposit operation with the exclusion of oxygen” and “under a protective gas atmosphere”, respectively.
However, out of several practical reasons, a two-step thermal polymerization is very much undesired by PCB manufacturers. First, commercially available solder masks and solder mask materials are not characterized and specified for a two-step thermal polymerization. Since solder masks are typically fully polymerized in a single thermal treating step, no individual temperature range is defined for a respective pre- and post-polymerization step. Such additional specifications have to be carried out and determined each individually for each solder mask.
Second, a PCB with an only pre-polymerized solder mask, handled for example in an automated soldering process is vulnerable to scratches and scares caused by mechanically hard/solid components, such as a conveyor roller. The less polymerized (hardened) the solder mask is the higher is the risk of damaging the solder mask during the automated soldering process. This easily increases the number of defective goods. Furthermore, solder masks are becoming thinner and thinner (e.g. having a layer thickness of 20 μm or less) and, thus, are more susceptible to mechanical defects.
Third, a not fully polymerized solder mask tends to increase its volume due to the incorporation of water or other solvents. Such a so called post swelling is undesired (and usually takes place only if the solder mask is not fully polymerized), in particular for fine line circuitry, because of the dimensional changes and delocalizations of the openings in the solder mask.
Fourth, a two-step thermal polymerization usually requires a first heating unit for the pre-polymerization and an additional heating unit for the post-polymerization. However, such a heating unit arrangement is undesired because of additional costs and streamlining reasons.
WO 2009/125845 A1 discloses a method for manufacturing a flexible wiring board. In the method, heat treatment for reacting a polyimide compound constituting a polyimide layer, on which a predetermined pattern has been formed by exposure and alkali developing treatment, with a crosslinking agent is performed under an atmosphere having an oxygen concentration of not more than 1% by volume. The heat treatment in the very low oxygen content of not more than 1% by volume can prevent a change in color in not only an exposed surface of a conductor circuit but also an interface between a conductor and the polyimide layer and can prevent the occurrence of the lifting of the polyimide layer after plating.
In order to additionally increase adhesion between a solder mask and a surface of a copper circuitry various copper surface treatments (also sometimes called peel strength increasing surface treatments) are known.
In WO 2012/005722 A1, claim 1 a “method of treating a metal surface to promote adhesion between the metal surface and an organic material [is disclosed] characterized in that: a metal oxide layer is formed on the metal surface, and formation of the metal oxide layer is controlled by a self-limiting reaction between the metal oxide and a surface modifier compound”. This reaction includes an oxidation and subsequent reduction reaction of the copper surface. According to examples 2 and 5 this reaction is applied to a build-up layer and a solder mask, respectively. However, polymerization of said build-up layer and solder mask is carried out in the presence of ambient air.
Similar to solder mask applications, delamination tendencies and adhesion problems occur and have been observed for build-up layer applications during multi-layer PCB manufacturing. Simply put, a multi-layer PCB is formed by alternately stacking build-up layers and copper circuitries. The build-up layers electrically insulate the copper circuitries from one another and furthermore provide stability. However, vias are formed at well defined positions, electrically connecting a circuitry of one layer with one or more than one circuitries of other layers. Such vias are formed for example by use of a laser, plasma, photo methods or conventional drilling.
Usually, a build-up layer is vacuum laminated onto a respective non-conductive substrate/layer and covers the copper circuitry of said substrate/layer. The build-up layer is subsequently pressed and subjected to a first thermal treating step to obtain a sufficiently pre-polymerized build-up layer. A temperature in the range from 170° C. to 200° C. is usually applied for approximately 30 minutes. It is common to apply this thermal treating in an oven with hot, clean air, i.e. in the presence of molecular oxygen.
Subsequent to that first thermal treating step, the pre-polymerized build-up layer is in a condition for patterning, in particular for via formation. Afterwards, the surface of the build-up layer and via surfaces are usually desmeared and basically in condition for forming the next copper circuitry in the multi-layer PCB.
The process of forming the next copper circuitry is carried out, for example, by means of (i) a subtractive process usually starting by laminating a copper foil, (ii) a semi-additive process (SAP) or advanced modified semi-additive process (AMSAP) typically starting with an electrolessly deposited copper layer, and (iii) a full-additive process (FAP) starting with forming a temporarily, structured photo resist layer for selectively depositing copper. Of these processes, the SAP is typically applied, in particular for manufacturing high density PCBs.
In the SAP, a copper layer is electrolessly deposited onto the build-up layer surface prior to a second thermal treating step usually carried out at a temperature in the range from 100° C. to 150° C. for at least 30 minutes (also often referred to as drying step). In this thermal treating step heat is applied to a pre-polymerized, copper plated build-up layer.
In a next step, a temporary, photo sensitive layer of non-conductive material (photo resist layer) is formed onto the layer of electrolessly deposited copper, subsequently patterned to form openings, and cured. Afterwards, additional copper is deposited into the openings.
In a last step, the temporary, photo sensitive layer and residual copper layer are removed. As a result, the copper circuitry of the next layer is formed.
It is essential that no delamination occurs during the life time of a multi-layer PCB. Therefore, it is desired that the adhesion between a copper circuitry and a build-up layer is as high as possible and remains during the life time of a PCB.
According to our own experiments, build-up layers sometimes suffer the disadvantage that insufficient adhesion results in delamination at the interface between copper surface and build-up layer, blisters are formed after stress testing, and wedge voids are formed in the proximity of blind micro vias (BMVs).